1. Field of the Invention
The present invention relates to an improvement in a pasted type lead-acid battery. More particularly, the invention relates to a method for producing a pasted type lead-acid battery plate.
2. Description of the Related Art
A pasted type lead-acid battery has a superior discharge capacity and especially a high rate discharge capacity when compared to a tubular type lead-acid battery. Therefore, these batteries have been extensively used as a drive power source in electric vehicle and the like. However, if deep discharge is repeatedly carried out, the service life of a pasted type lead-acid battery is shorter than that of a tubular type lead-acid battery. Furthermore, in a pasted type lead-acid battery, the weight of certain components, such as its grids which do not relate to the charge or discharge reaction, is reduced in order to increase the energy density of the battery. As a result, the structural supporting capability of the grid for the active material is decreased and the battery's service life is reduced accordingly. On the other hand, improvement of active material utilization may effectively increase the energy density but, unfortunately, the improvement cannot be achieved without reducing the service life of the battery for cycle use. Thus, if the energy density of a pasted type lead-acid battery is increased, then the service life in cycle use correspondingly decreases.
The service life in cycle use of a pasted type lead-acid battery is, in general, primarily dependent on the service life of the positive plates. Accordingly, in order to increase the service life of a pasted type lead-acid battery, it is essential that the service life of the positive plates be increased. Capacity loss of the pasted type positive plates during a charge-discharge cycle is attributed to the fact that the active material is softened and accordingly sheds. That is, the volume of the positive plate active material (PbO2) is changed by charge and discharge. More specifically, when the active material PbO2 is changed into PbSO4 through discharge, the molecular volume increases by a factor of 1.92. In contrast, during the charge PbSO4 changes to PbO2 and the volume of the material contracts by a factor of 1/1.92. However, it should be noted that the volume change in the active material layer due to charge and discharge is not reversible. In other words, as the charge and discharge is repeated, the plate is gradually expanded as a result of which large pores or voids are formed in the active material and the plate becomes more porous. As the porosity increases, the cohesion of the active material particles is gradually lowered thereby reducing electrical contact to the active material particles which decreases the capacity of the positive plates. In this condition, the active material layer is softened, and shedding of the active material particles from the plate results. This causes the successive degradation of the positive plate during deep charge and discharge cycle use.
In order to develop an electric vehicle which has an acceptable performance and is economical, it is essential to provide a lead-acid battery which has a high energy and power density and has a long cyclic charge and discharge life. In order to accomplish this, it is necessary to provide a pasted type positive plate which has a long service life.
In order to improve the service life for cycle use of a pasted type lead-acid battery, it is necessary to prevent structural change of the positive active material due to charge and discharge, specifically, the expansion thereof. There are a variety of currently available techniques for preventing the expansion of the active material. In one of the known techniques which can be applied to the pasted type positive plate, cloth made of glass or synthetic fibers having an acid resistance is wrapped around the surface of the plate or it is placed over the surface of the plate so as to apply pressure to the active material surface. In another technique, a bag is formed using such a cloth, and the positive plate is disposed inside the bag similar to the tubular type plate. These techniques may be effective in preventing shedding of the positive active material particles from the plate when the cohesion of the positive active material particles is reduced. However, these techniques do little to prevent the expansion of the active material and, accordingly, cannot effectively increase the service life of the battery as is desired.
In a typical conventional technique, a porous material such as a glass mat having some flexibility is disposed under pressure in contact with the surface of the positive plate. With this technique, a pressure of 5 to 20 kg/dm.sup.2 is typically applied to an assembled element in the dry state. The service life of a pasted type lead-acid battery using such a glass mat is longer than that of a pasted type lead-acid battery without a glass mat. However, it is still much shorter than that of tubular type lead-acid battery. Thus, it can be understood that the deep charge and discharge life cannot be sufficiently increased merely by using a glass mat.
Glass and paper mats have found a usefulness in the manufacturing of the lead-oxide battery plate. The use of paper as a means to improve the manufacturing process for applying lead oxide past to the grid of a lead-acid battery plate is well known in the lead-acid battery industry. A number of nonwovens have been evaluated as substitutes for the light-weight, low-cost pasting paper that is widely used, generally without success. Recently, a light-weight, low-cost chemically-resistant glass mat nonwoven has delivered sufficient processing improvements to merit adoption by a major battery manufacture. In addition to the processing benefits derived from the use of a pasting paper, the chemically-resistant glass mat reinforces the lead oxide paste, enabling it to better withstand the expansive and contractive stresses incurred during discharge and recharge, thereby increasing the life of the battery, as discussed above. However, improvement in the manufacturing process and the performance of lease oxide battery plates and the overall battery are continually sought.